This invention relates to the field of asphalt mixing devices. More specifically, the invention relates to the field of rotating drum asphalt mixers which combine virgin aggregate with fresh asphalt and, in some cases, combine these virgin materials with recycled or reclaimed pavement material obtained by breaking up all or a layer of an existing asphalt pavement.
Drum mixers of the type referred to receive virgin aggregate at an imput end near a heat source. This aggregate is heated to a high temperature for drying purposes. The heated aggregate is subsequently combined with fresh asphalt in either liquid or solid form. In some cases, it is desirable to add recycled pavement material which of course includes old asphalt as well as old aggregate. The asphalt and recycled material containing asphalt are introduced into the drum at an intermediate point where the temperatures are lower. The introduction of these materials in such an area is necessary because exposure of the asphalt (either fresh or as part of the recycled material) to high temperatures causes hydrocarbon smoke emissions of significant proportions.
Drum gas temperature is dependent on several factors, such as the proportion of virgin aggregate to recycled paving material, moisture content of any recycled material added, combustion gas temperature, mix temperature, and physical characteristics of the drum (e.g., length, diameter, rate of heat loss, etc.). If the temperature of the gases in the drum is excessive in the area where fresh asphalt and/or asphalt contained in recycled materials are added to the drum, undesirable amounts of smoke are generated.
One technique employed to reduce the smoke emissions under the foregoing operating conditions is to introduce excess combustion air into the drum at the input end of the mixer adjacent the burner; this procedure results in lower gas temperatures throughout the drum which would indicate a possible solution to the smoke problem. However, the lower temperatures in the first zone where the virgin aggregate is heated results in relatively low virgin aggregate temperatures and thus insufficient drying; this inadequate drying of the aggregate is detrimental to the efficient operation of the drum mixer and to the quality of the resulting product. For maximum efficiency, it is desirable to operate the gas or fuel oil burner (used to heat the drum) at the maximum combustion temperature to produce extremely hot gases which provide maximum heat transfer to the virgin aggregate. Such operation reduces the amount of virgin aggregate needed to produce a satisfactory product but, of course, this operation at high temperatures aggravates the smoke problem.
It is accordingly an object of the present invention to provide a method and apparatus for making an asphalt paving material using virgin aggregate, fresh asphalt and, if desired, recycled pavement materials, whereby the virgin aggregate in a drum mixer can be heated to a high temperature with maximum heat transfer while maintaining hydrocarbon smoke emissions from the asphalt at an acceptable level.
Another object of the invention is to provide a method and apparatus for cooling a selected portion of an asphalt drum mixer at the area or zone of entry of the asphalt, either in fresh or recycled form, to prevent excessive hydrocarbon smoke emissions.
It is also an object of the invention to provide a method and apparatus for injecting a cooling fluid into a drum mixer for asphalt to reduce the gas temperature at a selected portion thereof.
Other objects and advantages of the invention will be apparent from the remaining portion of the specification.
As indicated, drum mixers for use in preparing hot asphalt paving compositions are known. It is also known to provide asphalt mixers in which asphalt and/or recycled materials are added to the drum mixer at different locations. Drum mixers conventionally employ a gas or oil burner at an input end; these drums rotate and include internal flights for lifting the material and allowing it to drop through the hot gases as the material proceeds from the input end to a discharge end of the drum.
The present invention has particular, although not exclusive, application to drum mixers of the type adapted to utilize recycled pavement material. Because of the high cost of virgin aggregate and asphalt liquid, and because of the desirability of conserving natural resources, it is preferable to use old pavement materials whenever practical. If the asphalt in this recycled material is heated to excessive temperatures, significant amounts of hydrocarbon smoke are generated, presenting an obvious pollution problem. For that reason, the recycled material is generally admitted to the drum downstream from the input end at a substantial distance from the burner. Sometimes this physical spacing is sufficient to prevent the formation of excess amounts of smoke where the mix includes a large proportion of virgin aggregate, when the mix temperature is maintained at a relatively low level and when the combustion temperature is not too high.
For reasons of economy and efficiency, it is often desirable to utilize a high proportion of recycled material in relation to the proportion of virgin aggregate. When the amount of recycled material exceeds the amount of virgin aggregate, the smoke generated becomes excessive due to overheating of the asphalt admitted as part of the recycled materials.
Even when recycled material is not being used, the problem of excessive smoke emissions in the use of drum mixers is often present. The use of drum mixers in the making of asphalt paving material is a relatively new technology. As advances are made in this technology, it is believed that higher and higher combustion temperatures will be made possible in the interest of improving efficiency in drying the virgin aggregate when relatively high mix temperatures are required. Needless to say, these increased temperatures in the drying zone for the virgin aggregate will bring about smoke emission problems since the temperature in the drum where the fresh asphalt material is received will be high enough to generate an undesirable amount of smoke. The present invention provides a solution to this problem and permits maximum drying temperatures when using only virgin aggregate and only fresh asphalt.
It has been discovered that this undesirable smoke emission in the applications just described can be significantly reduced, if not entirely elminated, by the controlled admission of a selected amount of cooling fluid into the drum adjacent, and preferably just upstream of, the area or "zone" where the asphalt and/or recycled material is admitted to the drum. According to the present invention, this cooling is accomplished by forcing ambient air into the drum, or by water cooling, although the former is preferred. Control of the volume of air introduced can be accomplished by means of a variable speed fan or a damper arrangement. In the case of a water system, a variable flow water pump supplying water to a heat exchanger may be employed.
Introduction of cooling air into the mixing drum is accomplished, in the preferred embodiment illustrated in the drawings, by means of an axially located pipe which brings ambient air from the discharge end of the mixer to a location in the drum just upstream of the hopper through which recycled material is introduced. In the water-cooled embodiment, not illustrated herein, a heat exchanger could be similarly positioned upstream of the point of introduction of the recycled material to reduce the temperature of the gases.